This invention relates to polycrystalline abrasive bodies and materials, and to a method of making them.
Abrasive compacts are used extensively in cutting, milling, grinding, drilling and other abrasive operations. They generally contain ultrahard abrasive particles dispersed in a second phase matrix. The matrix may be metallic or ceramic or a cermet. The ultrahard abrasive particles may be diamond, cubic boron nitride (cBN), silicon carbide or silicon nitride and the like. These particles may be bonded to each other during the high pressure and high temperature compact manufacturing process generally used, forming a polycrystalline mass, or may be bonded via the matrix of second phase material(s) to form a polycrystalline mass. Such bodies are generally known as polycrystalline diamond or polycrystalline cubic boron nitride, where they contain diamond or cBN as the ultrahard abrasive, respectively.
Examples of diamond and cubic boron nitride abrasive compacts are described in U.S. Pat. Nos. 3,745,623; 3,767,371; 3,743,489; 4,334,928; 5,466,642 and 5,328,875.
For example, U.S. Pat. No. 4,334,928 teaches a sintered compact for use in a tool consisting essentially of 80 to 20 volume percent of high pressure form boron nitride; and the balance being a matrix of at least one binder compound material selected from the group consisting of a carbide, a nitride, a carbonitride, a boride and a silicide of a IVa or a Va transition metal of the periodic table, mixtures thereof and their solid solution compounds. The matrix forms a continuous bonding structure in a sintered body with the high pressure boron nitride interspersed within a continuous matrix. The methods outlined in this patent all involve combining the desired materials using mechanical milling/mixing techniques such as ball milling, mortars and the like.
In U.S. Pat. No. 5,466,642 it is taught that a wear resistant cBN-based cutting tool, superior in toughness, includes a specified amount of at least one of a Ti carbide/nitride component, a compound including at least one of Ti and Al, tungsten carbide, Al2O3, and the balance being cBN and incidental impurities. The method of manufacture as described involves wet blending in a ball mill. The incidental impurities mainly result from material abraded from the mill balls and body.
In U.S. Pat. No. 5,328,875 a PCBN ceramic comprising a composition having a bonding phase constituent component, a dispersed phase constituent component, and unavoidable impurities to provide a high strength ceramic for cutting tools of high toughness and high resistance to wear and chipping is claimed. The bonding phase constituent component has one or more of titanium and aluminum carbide, nitride and carbonitride compounds including oxygen, and 20% to 48% by volume of decomposed reaction phase cubic crystal boron nitride. The dispersed phase constituent component comprises cubic crystal boron nitride, and the decomposed reaction phase comprises one or more of titanium carbide, titanium nitride and titanium carbonitride, and one or more of aluminum oxide and aluminum nitride, as well as titanium boride. The crystal grain sizes in the bonding phase comprising the decomposed reaction phase, and the crystal grain sizes in the dispersed phase comprising the cubic boron nitride are all said to be less than 1 micron. The titanium and aluminum carbide compound is preferably Ti2-3AlC, the titanium and aluminum nitride compound is substantially Ti2-3AlN, and the titanium and aluminum carbonitride compound is substantially Ti2-3AlCN. The decomposed reaction phase substantially comprises one or more of TiC, TiN, TiCN, Al2O3, AlN and TiB2. The described method of manufacture involves milling and mixing the desired component particulate materials in a wet ball mill.
Some significant problems arise with the methods of the prior art. The general methods involving mechanical milling and mixing procedures in order to combine the desired starting materials lead to unavoidable comminution and crushing of said components. This in turn causes a wide spread of particle sizes of the often complex and manifold components to be generated with a resultant lack of homogeneity of the components. This inhomogeneity leads to an inability to accurately determine and control the phase structure of the final material after sintering and in turn the true potential of the material as a cutting tool or the like cannot be exploited. Such materials can also exhibit poor characteristics in applications, which result from an inadequate dispersion and homogeneity of the constituents.
Moreover these procedures are inappropriate as the particle sizes of the desired starting constituents become finer, in particular for submicron particulate materials and more particularly for nano-sized component materials, due to significant difficulties in dispersion. Use of these procedures thus imposes limitations on making composite materials with homogeneous submicron and nano-sized phases.
Further it is impossible to mill ultrahard abrasive particulates without to a greater or lesser extent abrading the mill balls, rods and mill body materials. The material so generated by this abrasion necessarily contaminates the mix of desired components with either undesirable material or, if that material could be considered as desirable, then it will be introduced in an uncontrollable and variable way. This contamination is particularly prevalent when high energy milling techniques are employed in an attempt to use submicron and nano-sized starting constituent materials. During the life of milling bodies, balls and rods the inescapable abrasion leads to progressive changes in dimensions and surface texture of these items which leads to a progressive change in their milling, mixing and comminution behaviour. These changes lead to further variability in the dispersion, homogeneity, and degree of contamination of the materials being combined and so, in turn, variability in the structure, properties and behavior in application of the finally produced composite materials and tools. Moreover submicron and nano-grain sized materials are particularly prone to these problems and difficult to make with such methods.
Milling and mixing procedures also tend to damage and break up fibers, whiskers and in general high aspect ratio particulate materials which might be added to modify the mechanical properties of the desired composite, usually for toughness enhancement and thus defeat the object thereby.
There are examples in the prior art where milling and mixing techniques are not predominantly employed. For example, it is taught in U.S. Pat. No. 5,211,726 that granules of cBN or diamond, of a range of sizes from fine, about 0.1 micron, to coarse, about 1 mm, may be coated in one or more layers of active coating and these coated entities sintered at a pressure and temperature to yield multigrain abrasive compacts. The methods of coating are restricted to chemical vapor deposition (CVD) techniques, for coating multigrained granules of a specific type of cBN material from about 50 micron to about 1 mm in size.
EP 0 577 375 also teaches a method for producing abrasive compacts utilizing diamond or cBN as ultrahard components, whereby coatings of refractory oxides, nitrides and carbides are deposited onto the diamond or cBN and the coatings sintered at temperatures and pressures where the diamond and cBN are expected to be thermodynamically stable. The method of coating disclosed is chemical vapour deposition involving diamond or cBN particles to be coated in the sizes 20 to 40 microns.
U.S. Pat. No. 5,536,485 discloses a method whereby a diamond or cBN sinter can be produced by first coating the diamond or cBN particles in gaseous or vapour environments followed by sintering said coated particles at temperature and pressure conditions where diamond and cBN may be both thermodynamically stable and thermodynamically metastable.
Much of the prior art concerning materials where cBN is the ultrahard component depend upon reactions with metals such as aluminium, titanium or silicon, which are capable when molten of wetting the cBN, significantly reacting with the cBN and causing it's partial decomposition. These approaches therefore lead to materials that have the resultant decomposition phases incorporated into the complex microstructure of the resultant material. Necessarily complex borides, nitrides and boronitrides of the reacting metals will be present, often in inhomogeneous distributions with the other phases introduced. This tends to limit the materials that can be produced to those possible by the respective reactions and to excessively complex structures.